Electric arc welding is a well established process, and the present state of the art reflects some 50 or 60 years of progress in most of the hardware and in many of the techniques presently being used. However, the technique of starting an arc and the contribution of the welding power source to this part of the welding process has not changed significantly since the early days of the art.
For reasons of safety, welding power sources used with the various manually applied versions of the electric arc welding process have had their open circuit voltage limited to 100 volts, or less, and in order to start the welding arc, the welder has, since the beginning of the art, scratched or pecked the welding electrode lightly against the workpiece, which creates a spark of sufficient intensity to ionize the area between the end of the electrode and the workpiece. This establishes a low resistance path for the welding power to flow through, and the result is an instantaneous welding arc of full potential which melts both the electrode and the workpiece opposite the electrode which one skilled in the art controls to produce a weld.
The starting of a welding arc appears simple enough to be of little significance. However, those skilled in the art readily acknowledge the difficulty associated with this seemingly simple action. When the welding electrode is scratched or pecked against the workpiece, to start an arc, very high current densities are involved; and if the electrode is not withdrawn quickly enough, a small weld will be formed between the electrode and the workpiece. In order to break this weld, the welder must vigorously bend the electrode from side to side while pulling at the same time. Frequently, this action damages the coating on the electrode enough to make the electrode unusable. Conversely, in an attempt to withdraw the electrode away from the workpiece quickly enough to avoid such welding to the workpiece, the welder often accelerates the withdrawal rate of the electrode from the workpiece such that he does not stop quickly enough and the arc pops out because the length of the conductive path from the electrode to the workpiece is too long to sustain an arc under starting conditions.
Frequently, when welders are welding in the vertical, overhead, or other difficult positions which are encountered routinely in shipyards or on large construction projects, such as oil refineries, power generating plants, chemical plants, or paper mills, welders do experience the aforementioned difficulties in the starting of an arc sufficiently often for it to be a very real problem. In fact, a large percentage of all weld defects originate at the arc starting points.
Another problem which exists when the welder is attempting to start an arc is off-target arc starts. In accordance with the prior art, off-target arc starts arise from the lack of a means of control by the welder over the full welding potential which exists immediately upon contact of the electrode with the workpiece. The problem is compounded by the following facts. The electrode extends out approximately 14" from an electrode holder, which averages around 8" in length and is held by the welder's hand. Since the rays emitted by the arc are harmful and the light created is very bright and intense, the welder's protective shield must be in place over his face for viewing the welding through a very dark lens. Consequently, the welder does not see the exact spot where the arc will start until it is established. Accordingly, many arc starts are considerably off-target, and the workpiece, or adjacent objects, such as electric conduits, instruments, or machine parts, are damaged to the extent that production delays and costly repairs result.
A further problem area of electric welding, but more directly related to the gas tungsten arc welding process where exceptionally high current densities are involved, is the loss of the point of the tungsten electrode when an arc is started by scratching or pecking against the workpiece (the method most commonly used throughout the construction industry) since a piece of the point often welds itself to the workpiece and breaks off and becomes deposited in the weld. This is very undesirable since not only is the welder required to regrind a point on the tungsten, but the weld may fail to pass X-ray examination, necessitating costly repairs. Another source of tungsten deposits in the weld metal, and perhaps the greatest cause of this problem, is the fact that a welder cannot always position himself in relation to the weld to accurately discern the true distance from the point of the tungsten to the molten weld metal, so the welder often accidentally dips the point of the tungsten into the molten weld metal and inadvertently deposits a small piece of the point in the weld.
The aforementioned problems have been recognized by others, and there are many various approaches toward a solution to many of these problems, but apparently no acceptable answer has been found that is simple enough and economical enough to be adapted on a wide scale to shielded metal arc welding or to the field applications of gas tungsten arc welding.
There are several patents present in the art pertaining to welding systems, the most pertinent being:
______________________________________ U.S. Pat. No. Title ______________________________________ 3,133,187 Touch Starting of Power Arcs 3,774,007 Welding Apparatus 3,051,829 Electric Arc Torch Starting 3,459,996 Starting Circuit For Direct Current Art Welding 3,174,027 Pilot Arc Starting-Arc Working Systems 8,876,855 Tungsten Inert Gas Arc Striking Device 3,637,974 Switching Arrangement For The Stabilization And Ignition Of Welding Arcs And The Like 3,609,290 Electric Arc Control System 2,504,837 Welding Rod Holder 3,335,317 Arc Starting Circuit For Welding Systems ______________________________________
It is the object of this invention to provide an arc welding control system which effectively solves the problems discussed and provides a degree of control over the welding process which enables welders of all skill levels to perform their art to a higher degree of excellence. This system senses the welding arc as a variable resistance wherein varying the length of the arc produces an equally varying and proportional change in the resistance across the arc. Conversely, a change in the resistance across the arc produces a proportional change in the voltage across the arc which is in keeping with Ohms Law of Proportionality. So, if an 1/8" arc length represented a 20 volt arc, an arc length of more than 1/8" would require somewhat more than 20 volts, and an arc length of less than 1/8" would require somewhat less than 20 volts. It is therefore apparent that a control system which senses the voltage of an arc welding circuit may be designed to effect a switching signal when the voltage of that welding circuit goes above or below certain preselected values.